1. Field of the Invention:
This invention relates to a metal powder used for making a sintered metal product, particularly one which is mixed with a binder to form a composition to be formed by injection molding or otherwise into an intermediate molded product to be sintered, and to a process for making a sintered metal product.
2. Description of the Prior Art:
It has hitherto been usual to make a sintered metal product by pressing a metal powder to form a compacted body and sintering it. It has, however, been very difficult to make by such a process any sintered product having a complicated three-dimensional shape, a reduced wall thickness, or a knife edge.
Modified processes have been proposed to overcome the difficulty as hereinabove stated. According to the disclosure of U.S. Pat. Nos. 4,197,118, 4,305,756, 4,404,166, 4,415,528, 4,445,936, 4,602,953, 4,765,950, a mixture comprising a metal powder having an average particle diameter not exceeding 10 microns and an appropriate binder is formed by injection molding or otherwise into an intermediate molded product, the binder is removed from it by heating or solvent extraction, and the intermediate product is sintered. These processes can make a product having a high sintered density. They, however, have a number of drawbacks, too, as they require the use of a large amount of binder. The removal of the binder requires a long time. The heavy shrinkage of the material which occurs when it is sintered results in a sintered product having a low degree of dimensional accuracy. Moreover, the mixture which is employed is expensive.
The economical disadvantage as hereinabove pointed out can be improved by the use of a metal powder having an average particle diameter exceeding 10 microns. It, however, presents a number of problems, too. Such a powder yields a product having a low sintered density. Its mixture with a binder is less easy to mold by injection or otherwise into an intermediate product. Moreover, the intermediate product lowers its strength and even fails to retain its shape, when the binder is removed from it.